1. Field of the Invention
The present invention relates to the art of electrometallurgy and is specifically concerned with a method and apparatus for the electroslag remelting process.
The invention is particularly applicable to producing large ingots.
The term "melting operation", as used throughout the specification and claims means the process of making one ingot by the electroslag remelting method.
The term "melting rate" is used to suggest the rate of molten metal accumulation in a melting container in the course of electroslag remelting of one or more consumable electrodes, expressed through the linear speed of movement of the metal bath surface along the height of the melting container.
The term "metal bath depth" is used to suggest the distance from the metal bath bottom, i.e. the solid-liquid interface in the ingot, to the metal bath surface, measured along the vertical axis of symmetry of the melting container at the time moment under consideration.
2. Description of the Prior Art
When making a large solid-section ingot by the electroslag remelting process, it is difficult to attain a compact fine-grained metal structure throughout the ingot volume, since in the course of crystallization due to decline in the cooling effect of the bottom plate with the increase in height, the center portion of the metal bath deepens considerably as the ingot grows, and the bath bottom acquires the shape of a deep steep-sided funnel; this favors development of segregation effects and entrapment of non-metallic inclusions in the center portion of the ingot.
Various attempts to control crystallization in the process of ingot making by exposing the metal bath to an electromagnetic field, ultrasound oscillations, or mechanical vibration which ensure breakdown of large crystals in the course of their growth have been undertaken for levelling the metal bath bottom with the aim of obtaining a sound metal structure.
However, said methods yield a stable effect only in making ingots of not more than 1000 mm in diameter and 2.5 m in height. Applying the methods for larger ingots entails great power losses and fails to yield any adequate results.
There has recently been developed a power modulation technique employed in particular in the method for producing an ingot by electroslag remelting, disclosed in U.K. Pat. No. 1,421,393, Class H2H, 1976.
This method contemplates connecting each electrode or an electrode group to a separate power source. After steady-state remelting conditions have been attained, the current and voltage fed to each electrode or to each electrode group are alternately decreased and increased so that the power of the current supplied alternately reaches the maximum and the minimum. This procedure provides for recurrent-progressive or reciprocating motion of the maximum heat point in the slag bath, which allows a homogeneous, compact, and fine-grained structure or remelted metal to be obtained.
The above method is applicable for large-section ingots, but yields a marked effect only with relatively short ones. Besides, its application is confined to a multielectrode apparatus.
The method calls for cumbersome, complicated, and costly equipment including several power sources, means for switching these, a large number of current leads interfering with access to the working elements of the apparatus and causing considerable electric power losses.
Well known in the art are electroslag remelting apparatus for producing hollow ingots, which provide for a sound metal structure owing to the presence in the melting container of a cooled body needed to form a hollow in the ingot.
There has been provided, for example, an electroslag remelting apparatus for making hollow ingots, comprising a cooled body in the form of a molding core, linked to a lifting device, in a melting container formed by a mold and a bottom plate (see, e.g., U.S. Pat. No. 3,807,487, Class 164-252, 1974).
In a similar apparatus with a movable mold resting in its lowermost position on a bottom plate to form therewith a melting container, the core is linked to the lifting device not directly but through the movable mold coupled with the lifting device, the core being fixedly attached to the mold (see B. E. Paton, V. R. Demchenko et al. "Matematicheskoe opisanie protsessa zatverdevania pologo elektroshlakovogo slitka" in "Rafinirujuschie pereplavy", edited by B. E. Paton, Member of A. Sc. of the USSR, No. 2, p. 35, FIG. 2; Kiev, 1975).
The core in the above apparatus is provided with supporting members allowing the core to be secured to the top end face of the movable mold and with an extension projecting downwardly from the end face for a length exceeding that of the movable mold.
The apparatus of the above type also generally comprises a liquid metal level detector which is arranged in the wall of the movable mold for producing commands which provide the controls for controlling the vertical movement of the movable mold in accordance with the movement of the slag-metal interface relative to the detector operation.
The above-described apparatus are unfit for making solid-section ingots, since the cooled body (core) is arranged therein so as to prevent the molten metal of consumable electrodes from penetration into the core location zone when the core is moved, with the result that a hollow is formed in the ingot.
A considerable core-to-molten slag contact area results in great heat losses in the apparatus and hence in an additional power consumption.